Given a random state \(|\psi \rangle \) , Page's theorem [1]} tells us that for sufficiently small \(k\) , \(l\)  and large \(N\) , such that \(k+l\ll N/2\) ,
\(\langle \psi |\mathcal {O}_k^{\dagger }\mathcal {O}_l|\psi \rangle \approx |\mathcal {H}|^{-1} \textrm {Tr}(\mathcal {O}_k^{\dagger }\mathcal {O}_l)=(\mathcal {O}_k|\mathcal {O}_l),\) 
